This invention relates to warp (adjustment) coils for adjusting the frequency of crystal controlled oscillators in the VHF and UHF areas, for example, and it is an object of the invention to provide an improved warp coil of this nature. In such oscillators, the circuits utilizing the piezoelectric crystals need adjustability because the crystals themselves, after manufacture, do not have the precisely defined frequency required.
Prior solutions to this problem have included the use of a toroidal inductor plus a warp coil in series with the piezoelectric crystal. While this solution solved the problem within some limits, it lacked the necessary sensitivity and was expensive because of the cost of the toroidal inductor and the warp coil. In this prior solution the toroidal inductor usually had a ferrite core and could thus provide sufficient inductance for producing the necessary coarse adjustment, but an additional series warp coil was necessary to produce the fine adjustment. In these fine adjustment coils there tended to be excessive inductance change per unit turn of the core in the coil to have the necessary decreased sensitivity for accurate adjustment of the final desired frequency unless specially spaced wound coils were used.